IT is interesting to examine Mr. O'Farrell's plan for defeating the conspiracy to conceal the change of length of the rod; but the resourcefulness of the conspirators is equal to the occasion. A compensation will take place in connection with the reflection of the light from the moving mirror. Light rebounds from a fixed mirror as though it were a billiard ball rebounding from a perfectly elastic cushion. If the cushion were moving with a great velocity the angle of rebound would naturally be modified. That is only an analogy, but it will perhaps show that we cannot apply the rules of elementary optics to the formation of images by a mirror moving through the æther. A mathematical discussion, on the basis of Huygens's principle, shows that a change of size of the image will be introduced which compensates for the change of size of the rod. It may be remarked that in order to deflect the ray from the horizontal to the vertical direction the mirror, although apparently inclined at 45° to the horizontal, would actually (in terms of the “real” space) be inclined at 26.6° (tan-1 1/2); this illustrates how the laws of reflection become modified in the conditions postulated.